Plant Growth and Development | Biology Notes for NEET/AIIMS/JIPMERClear Exam4.9 of 5

PLANT GROWTH AND DEVELOPMENT INTRODUCTION Growth is a characteristic feature of all living organisms. Growth is a vital proces...

PLANT GROWTH AND DEVELOPMENT

INTRODUCTION

Growth is a characteristic feature of all living organisms.

Growth is a vital process which brings about permanent and irreversible changes in any plant or its parts.

Growth is the final product of successful metabolism, i.e., during growth, anabolic process dominate over catabolic process.

Growth is diffused in animals but growth in plants is localised and irregular.

Development is a process in which cells change form & function to form the specialized tissues, organ & structure required during the life cycle of a plant. It commences with the first cell division after fertilization of the ovule & continues through seed development, seed germination, the development of the seedlings to the mature plant, flowering & production of next generation of ovules. It also includes the processes of cell death & senescence. During this period, a complex body organisation is formed that produces roots, leaves, branches, flowers, fruits & seeds & finally dies.

PLANT GROWTH

GROWTH IS GENERALLY INDETERMINATE

Plant growth is unique because plants retain the capacity for unlimited growth throughout their life. This ability of plants is due to the presence of meristems at certain locations in their body. The cells of such meristems have a capacity to divide and self perpetuate. These cells later form plant body when they lose the capacity of dividing.

Some plant structures are determinate while others are indeterminate. A determinate structures grows to a certain size and then stops, eventually undergoing senescence and death (e.g. leaves, flowers and fruits). On the other hand, the vegetative stems and roots are indeterminate structures. They grow by meristems that continuously replenish themselves. When an indeterminate vegetative meristem becomes reproductive (i.e., begins to form a flower), it becomes determinate.

There are three meristematic regions in plant i.e., apical, intercalary and lateral.

Apical meristems : These meristems are found at shoot and root apex. As a result of activity of these meristems plant increases in length. In angiosperms, and gymnosperms there is a group of meristematic cells but in bryophytes and pteridophytes there is a single tetrahedral cell found at the shoot apex.

Intercalary meristems : These meristems are found above the nodes. As a result of the activity of these meristems plants increase in length, e.g., Bambusa.

Lateral meristems : These meristems are made up of cells which divide in radial direction only. Cork cambium (phellogen) and vascular cambium are the examples of lateral meristems. Increase in girth of shoots and roots take place because of the activity of this cambium.

GROWTH IS MEASURABLE

Growth is a result of increase in the amount of protoplasm. It is difficult to measure the amount of protoplasm but growth is measured by a variety of parameters which is more or less proportional to the amount of protoplasm.

Growth in plants means increase in shape, size, weight and volume of a plant or plant part. Growth leads to increase in fresh weight, dry weight, length, area, volume and cell number and all these are controlled extrinsically (by environmental factors) and intrinsically (by nucleus and protoplasm).

PHASES OF GROWTH

Phase of cell formation : This phase is limited to the apex of shoot and root meristems. Cells of this region continuously divide and increase in number. The cells in this region are rich in protoplasm, possess large conspicuous nuclei and thin cellulosic walls.

Phase of cell elongation : This phase is found exactly below the cells of phase of cell formation. In this phase, small vacuoles are formed in which water and dissolved matter are collected and at the end, all vacuoles unite to form a big vacuole. A big vacuole is situated at the centre, nucleus and cytoplasm are situated as a thin layer at the internal layer of cell-wall, called primordial utricle.

Phase of cell maturation or differentiation phase : Cell differentiation following cell division and cell enlargement leads to the development of specialized mature tissue cell, e.g., some cells are differentiated into xylem tracheids and trachea and some others into sieve tubes and companion cells.

PLANT GROWTH RATE

Plant Growth Rate refers to increase in growth per unit time.

It is of two types :

ARITHMETIC GROWTH

From a dividing cell, two new cells are formed (by mitotic division), out of them one daughter cell continues to divide while other differentiates and matures (stops dividing) .

E.g. Root and shoot elongation at constant rate.

It is mathematically expressed as

where Lt = length at time 't'

L0 = length at time 'zero'

r = growth rate/elongation per unit time.

On plotting the growth against time, a linear curve is obtained.

GEOMETRIC / EXPONENTIAL GROWTH

From dividing cell (by mitotic division) both daughter cells retain the ability to divide and continue to do so.

On plotting the growth against time, a typical sigmoid or S-curve is obtained.

It has 3 phases :

Lag period phase : In lag period, the growth is slow. It represents the formative or cell division phase.

Log phase/Exponential phase : During this phase growth is maximum and most rapid. It represents cell elongation phase.

Steady State phase : It represents cell maturation phase.

Time taken in growth phases (mainly log phase) is called as grand period of growth.

Quantitative comparisons between growth of living system can be done by the following methods :

Absolute growth rate : Measurement and the comparison of total growth per unit time in plant or plant parts.

Relative growth rate : The growth of the given system per unit time expressed on a common basis i.e. per unit initial parameter in plant parts.

Relative growth rate is generally high in young developing plant parts.

MEASUREMENT OF GROWTH

The following methods are designed to measure growth in length.

Bose’s direct method : Measurement is done between two marked points by a scale at regular intervals.

Bose’s horizontal microscope

Bose’s crescograph : It magnifies growth by 10,000 times.

Auxanometers

Arc Auxanometer

Pfeffer’s Auxanometer

Micrometer Screw Auxanometer

EFFICIENCY INDEX (E.I.)

Growth can be measured by an increase in size or area of an organ of the plant (leaf, flower, fruit etc.) in a unit time and is called as efficiency index. E.I. may be same or different from species to species and organ to organ.

FACTORS AFFECTING PLANT GROWTH

Light : Light is involved in photosynthesis and determines the direction of shoot and root growth. Light controlled morphogenesis of plant is called photomorphogenesis. In absence of light plant exhibit etiolation.

Temperature : Optimum temp. for growth is 20-35°C. temp. Above 45°C damages the protoplasm and growth is retarted. Effect of low temperature on flowering is called vernalization.

Water : Water maintains the turgidity of cell, which is essential for growth. TP is important for growth. In order for a cell to grow ψw must not be allowed to reach zero. Water is essential for the enzyme activity in protoplasm.

Oxygen : It is necessary for cell respiration.

Mineral nutrients : All essential elements are compulsory for growth and metabolism.

Pollutants : Several pollutants such as automobile exhaust, peroxyacetyl nitrate (PAN), pesticides etc. have detrimental effect on plant growth. Citrus and Gladiolus are very sensitive to fluorides. Poor growth of tobacco is observed in regions where ozone concentration is high.

Carbon dioxide : CO2 is essential for photosynthesis and hence it is essential for nutrition also. Due to change in photosynthetic rate, with the increase or decrease in CO2 concentration, the plant growth is also affected.

Nutrition : It provides the raw material for growth and differentiation and is a source of energy.

Growth regulators : These are manufactured by living protoplasm and are important internal growth regulators which are essential for growth and development.

DIFFERENTIATION, DE-DIFFERENTIATION AND RE-DIFFERENTIATION

Differentiation is applied to the qualitative differences between cells, tissues and organs.

During differentiation, cells undergo few to major anatomical and physiological changes both in their cell walls and protoplasm.

Dedifferentiation : In plants, the living differentiated cell which had lost the capacity of cell division are able to, regain the capacity of cell division under certain conditions called dedifferentiation. E.g. Formation of meristems intrafascicular cambium & cork from differentiated parenchyma cells.

Redifferentiation : The regain of differentiation capacity by losing the capacity of cell division for performing specific function by dedifferentiated cells.

DEVELOPMENT

Development is a term that includes all changes that an organism goes through during its life cycle from germination of the seeds to senescence.

In different phases of growth, plants follow different pathways and form different kinds of structures in response to environment. The ability to change under the influence of internal or external stimuli is called plasticity, e.g., heterophylly in cotton, coriander and larkspur. Occurrence of different types of structures on the same plant in different growth phases or under different environmental conditions is known as heterophylly. On the other hand, differences in the shape of leaves produced in air and those produced in water in buttercup also represents the heterophyllous development due to environment.

PLANT GROWTH REGULATORS

Plant hormone is a chemical substance which may be translocated to another region, for regulating one or more physiological reactions when present in low concentration.

All phytohormones are growth regulators but all growth regulators are not phytohormones.

Plant growth regulators are grouped into two categories based on the nature of their actions

They play an important role in plant response to wounds and stresses of biotic and abiotic origin.

They are involved in growth inhibiting activities such as dormancy and abscission.

AUXINS

Charles Darwin & F. Darwin (1880) were the first to study phototropism.

Darwin found that the light falling on the tip of Canary grass (Phalaris canariensis) coleoptile from one side causes some influence to be transmitted downwards due to which the coleoptiles curve towards the light.

Boysen & Jensen (1910) : They performed experiments on oat (Avena sativa) plant. In the first experiment, he removed the coleoptile tip and then replaced it on stump. On providing unilateral light the coleoptile tip gave positive curvature. They observed that if gelatin is inserted between the tip & stump is cut, then coleoptile bends towards the unilateral light. If mica were inserted then the coleoptile fails to show phototropism .

F.W. Went (1928) : Went isolated the growing tip of Avena sativa on agar plate & performed Agar-block experiment. He gave the name auxin to growth substance, thus credit of auxin discovery goes to F.W. Went. He also found that the curvature (bending) in Avena coleoptile is proportional, within limits to the amount of auxin in agar - block. This test was named as Avena curvature-test (Bioassay of auxin).

Went found that 27% auxin present on illuminated side and 57% on the dark side. (About 16% auxin lost on illuminated side and rest transferred to base). Transport of natural auxin is basipetal and polar type.

Kogl and Haagensmit (1931) : They isolated an active substance from urine of a pellagra patient which was called as Auxin - A or chemically auxenotriolic acid (C18H32O5).

Later a similar substance was isolated from corn grain oil and was named as auxin-B or auxenolonic acid. (C18H30O4).

Again Kogl, Erxleben and Haagen Smit 1934 - isolated another substance from human urine and it named as heteroauxin (IAA-C10H9O2N).

Auxin from Rhizopus was obtained by Thimann.

Auxin biosynthesis occurs by tryptophan amino acid in the presence of Zn++ ion.

Degradation or oxidation of auxin is by-

Enzymatic (by IAA oxidase) reaction.

Photo-oxidation reaction.

Now IBA (Indole Butyric acid) have also been isolated from plants (natural auxin) but IAA is most widely found auxin in plants.

The compounds which can be converted into auxins are called auxin precursors, whereas the compounds which inhibit the activity of auxin are called as antiauxins.

Natural auxins : These are naturally occurring auxins in plants and therefore are regarded as phytohormones. Indole 3-acetic acid (IAA) is the best known and universal auxin. It is found in all plants and fungi.

Natural auxins are synthesized in physiologically active parts of plants such as shoot apices, leaf primordia and developing seeds, buds (apex), embryos, from amino acid tryptophan. In root apices, they are synthesized in relatively very small amount. Auxins show polar movement. It is basipetal (from apex to base) in stem but acropetal (from root tip towards shoot) in the root. Auxins move slowly by diffusion from cell to cell and not through the vascular tissues. Auxins help in the elongation of both roots and shoots.

PHYSIOLOGICAL EFFECTS AND ITS APPLICATIONS

Apical dominance : In most higher plants, the growing apical bud inhibits the growth of the lateral (axillary) buds, a phenomenon called apical dominance. Removal of shoot tips (decapitation) usually results in the growth of lateral buds. It is widely applied in tea plantations, hedge-making.

Cell division & cell enlargement : Auxin is important in tissue culture and grafting. It stimulates division of intrafascicular cambium and also in healing of wounds.

Shortening of internodes : α-NAA induces the formation of dwarf shoot or spurs in apple, pear etc., thus number of fruits increases.

Flower initiation : Auxin is inhibitor of flowering but it promotes uniform flowering in pineapple and litchi plants.

Parthenocarpy : In fruits like orange, lemon, grapes, bananas etc. fruits can be developed without pollination and fertilization. They are seedless fruits. If stamens are removed from flower bud and auxin paste is applied to the stigma of the flower seedless fruits develop.

Selective weed killer : Weeds are the unwanted plants growing in a field along with a crop. These weeds cause competition for water, minerals, light and space and thus do not allow proper growth of the crop. Hence, these unwanted plants should be removed from the crop mechanically or by other methods. One of these methods is the spray of auxins like 2, 4-D.

Femaleness : Feminising effect in some plants.

Flower and Fruit thinning : Certain trees like mango form less number of fruits in alternate years. But auxin use can produce normal fruit crops every year.

When antiauxin (TIBA-Tri-Iodo-Benzoic acid) is sprayed on mature cotton field then cotton balls can be picked easily.

BIO-ASSAY

Bioassay means testing of substance for it’s activity in causing a growth response in a living plant or it’s parts.

Avena curvature test : Avena curvature test carried out by F.W. Went (1928), demonstrated the effect of auxins on plant growth while performing some experiments with oat (Avena sativa) coleoptile.

GIBBERELLIN

The cause of this disease is the fungi Gibberella (Ascomycetes) or Fusarium (Deuteromycetes) as confirmed by Kurosawa and Swada.

Yabuta and Sumiki 1938 were first to extract a crystalline substance from the Gibberella fungus, which they named as Gibberellin.

Gibberellin is acidic and possess a gibben ring structure that are able to overcome genetic dwarfism in plants.

100 types of gibberellins (GA1, GA2, GA3, ....... GA100) are known . GA3 [C19H26O6] is representative of all Gibberellins. First discovered Gibberellins from higher plants was GA1 (GA1 and GA20 are common GA’s of higher plants).

GA found in all group of plants (algae to angiosperms) but as a flowering hormone acts only in angiosperms.

Elongation of genetic dwarf plants : When gibberellins are applied to dwarf Maize, Pisum & Vicia faba, then they become tall. The rosette plant of sugarbeet indicate an extreme dwarfism, this habit can be eliminated by GA.

Parthenocarpy : Like auxin, exogenous use of GA also induces the formation of seedless fruits.

Substitution of cold treatment or vernalisation : In many plants low temperature (0–10°C) is essential for flowering. They form vegetative body in the first year and reproductive growth in the second year (for low temperature). But GA overcomes the requirement of low temperature and flowering can be done in the first year.

Breaking of dormancy : GA breaks the dormancy of seeds, buds and rhizomes.

BIO-ASSAY

Synthesis of α-amylase and proteases in germinating grains of cereals : Gibberellins also stimulate the synthesis of α-amylase and protease enzymes which are involved in the conversion of starch into sugar. The gibberellin produced in the embryo moves to the layer of aleurone granules where it stimulates the synthesis of α-amylase and protease enzymes. Protease converts an inactive β-amylase to the active form. The active β-amylase and α-amylase together digest starch to glucose which is mobilized to meet the metabolic demands of the embryo.

Dwarf pea and Maize test : Seeds of dwarf pea are allowed to germinate till just the emergence of plumule. GA solution is applied to some seedlings, others are kept as control. After 5 days, epicotyl length is measured. Increase in length of epicotyl over control seedlings is proportional to GA concentration.

CYTOKININS (CK)

Cytokinin was discovered by Miller when he was working in the lab. of Prof. Skoog on tobacco pith culture. He added the contents of an old DNA-bottle (Herring fish sperm DNA) to the culture medium & observed that the tobacco pith callus could grow for a longer period.

Miller isolated an active substance from autoclaved Herring sperm DNA, which stimulated cell division. He named this substance as kinetin.

The first natural cytokinin was identified and crystallized from immature corn grains by Letham and named as zeatin.

The most common cytokinin in plants are zeatin and isopentenyl adenine.

Cytokinin is a derivative of adenine base.

Root tips are major site of synthesis of cytokinin (by Mevalonic acid pathway).

Movement of cytokinin is polar & basipetal.

Coconut milk also performs activity like cytokinin, and is, thus, used in tissue culture.

PHYSIOLOGICAL EFFECTS AND ITS APPLICATIONS

Cell division (Characteristics function of cytokinin) & Cell enlargement : One of the most important biological effects of CK (cytokinin) on plants is the induction of cell division. This has been established in roots, anthers and in callus tissue.

Breaking the dormancy of seeds : Like GA, the dormancy of certain seeds can be broken by CK.

Seed germination : Seeds of parasite plant (Striga) can germinate in the absence of host by CK treatment.

Delay in senescence (Richmond Lang effect) : The ageing process of leaves is usually accompanied with the loss of chlorophyll and rapid catabolism. This is called as senescence. Senescence can be postponed by CK (increase in short life of plant parts).

Lignin biosynthesis.

Parthenocarpy in some fruits.

Cytokinin stimulates the conversion of immature plastids into functional chloroplasts.

Phloem conduction : stimulates mobilisation of nutrients.

Sex expression : Cytokinins causes femaleness in plants.

Flowering in SDP (also in some long day plants).

Induces stomatal opening.

BIO-ASSAY

Tobacco pith cell division test : Tobacco pith culture is divided into two weighted lots. One is supplied with cytokinin and the other is kept without it. After 3-5 weeks, increase of fresh weight of treated tissue over control is noted. It is a measure of stimulation of cell division and hence cytokinin activity.

Chlorophyll preservation (retention) test : Leaves are cut into equal sized discs with the help of a cutter. They are divided into two lots. One lot is provided with cytokinin. After 48-72 hours, leaf discs are compared for chlorophyll contents. Cytokinin retards chlorophyll degradation.

Soyabean and radish cotyledons cell division test : Excised cotyledons are measured and placed in test solution as well as ordinary water (as control). Enlargement of cotyledons indicates cytokinin activity.

ABSCISIC ACID (ABA C15H20O4)

During mid-1960s, three independent researches reported the purification and chemical characterisation of three different kinds of inhibitors : inhibitor-B, abscission II and dormin. Later all the three were proven to be chemically identical. It was named as abscisic acid (ABA).

Addicott and Okhuma (1963) obtained a substance from mature cotton fruits and named it as abscisin. (C15H20O4)

Warieng and Robinson isolated a growth inhibitor from old betula leaves and called it as dormin.

ABA is synthesized in old leaves, fruits and old parts by mevalonic acid pathway and oxidation of carotenes in chloroplasts.

ABA is also known as the stress hormone because it protects plants from adverse conditions like water stress. ABA increases tolerance of plants to various type of stresses.

Inhibition of cell division and cell elongation. It is working against cytokinin and auxin (anti-CK and anti-auxin).

Stomatal closing : ABA accumulates in high concentration in leaves which are wilting. This increased production of ABA stimulates stomatal closure by inhibiting the K+ uptake by guard cell and promoting the leakage of malic acid and hence prevents transpiration.

Delaying of flowering in LDP

Seed development : Abscisic plays major role in seed development and maturation, enabling seeds to withstand desiccation and to become dormant.

Inhibitor of seed germination : ABA is an inhibitor of synthesis of α-amylase. Thus, it inhibits the germination of seeds.

Geotropism in roots

Growth inhibition in Duckweed (Lemna)

BIO-ASSAY

Rice seedling growth inhibition test : Mohanty, Anjaneyulu and Sridhar (1979) used rice growth inhibition method to measure ABA like activity. The length of second leaf sheath after six days of growth is measured.

Inhibition of α-amylase synthesis in barley endosperm test : ABA inhibits the synthesis of α-amylase in the aleurone layers which is triggered by gibberellins. Goldschmidt and Monselise (1968) developed the bioassay method to estimate ABA activity by determining the extent of inhibition of α-amylase synthesis induced by treating barley seed endosperm with GA.

ETHYLENE

Cousins confirmed the release of a volatile substance from ripened oranges that hastened the ripening of stored unripened bananas. Later, this volatile substance was identified as ethylene, a gaseous plant growth regulator.

Biosynthesis of ethylene takes place by methionine amino acid. Ethylene is synthesized in large quantities by ripening fruits and senescent organs.

PHYSIOLOGICAL EFFECTS AND ITS APPLICATIONS

Flowering : Like auxins, ethylene retards flowering in most plants but increases flowering in pineapple like auxins.

Sex modification : Ethylene increases the number of female flowers and reduces the number of male flowers.

Abscission : Ethylene causes faster abscission of leaves and flowers.

Fruit ripening : Ethylene is a ripening agent. Ethylene is formed in large quantities in ripening fruits.

Fruits that respond to ethylene usually have a major increase in respiration just before ripening occurs. The increase in ethylene production at that time is often up to 100 times greater than the normal. The accompanying major increase in respiration is called climacteric and fruits that exhibit such phenomena are called climacteric fruits e.g. apple, banana, mango, tomato etc. Some fruits such as grapes, pineapple, water-melon are non-climacteric and do not respond in such a way to ethylene.

Ethephon : 2-chloroethyl phosphonic acid is a liquid from which ethylene gas released gradually, hence this substance is used for artificial ripening of fruits. Fruits so ripened are similar to normally ripened fruits, in colour and shape etc.

BIO-ASSAY

Triple pea test : Pratt and Biale (1944) developed this method for bioassay of ethylene which is based on the physiological effect of ethylene to cause –

Subapical thickening of stem

Reduction in the rate of elongation

Horizontal nutation (transverse geotropism) of stem in etiolated pea seedlings. In the presence of ethylene, epicotyls show increased growth in thickness and reduced rate of longitudinal and horizontal growth.

PHOTOPERIODISM

First of all Garner and Allard studied the “Effect of requirement of relative length of day (photoperiod) and night (dark phase) on flowering of plants and this process is known as photoperiodism”.

They experimented on a mutant variety of tobacco and soybean.

Critical period : Critical photoperiod is that continuous duration of light, which must not be exceeded in short day plants and should always be exceeded in long day plants in order to bring them to flower.

The plants are classified into three groups according to their photoperiods viz., long-day plants, short-day plants and day-neutral plants.

SDP (SHORT DAY PLANTS)

These plants flower on exposure to photoperiod equal to or shorter than their critical day length. They need a continuous (uninterrupted) dark period of flowering. Thus, SDP is appropriate to called as long night plants.

In SDP, the dark period is critical and must be continuous. If this dark period is interrupted even with a brief exposure of red light, the SDP will not flowers.

Pfr (phytochrome Far Red) or P730 phase : This is far-red light absorbing form, which induces flowering in LDP and then converts into Pr form. Absorption Range is 720-740 nm. Absorption peak is at 735 nm. The Pfr (Yellowish) form, gradually changed into Pr (Bluish) form in dark.

During the day, the Pfr form is accumulated in the plants which is inhibitory to flowering in SDP but stimulates flowering in LDP.

Phytochrome - Pfr (P730) is the active form which controls many photos physiological processes (e.g. seed germination) in plants.

Control of morphogenesis by light & phytochrome is called photomorphogenesis.

Photomorphogenesis in higher plants appears to be under control of one of three photoreceptors:

Phytochrome - which absorbs red and far red region of light.

Cryptochrome - which absorbs blue and UV-A (approx. 380nm) light.

UV-B photo-receptor - which absorbs UV-B (approx. 290 nm) light.

VERNALISATION

Vernalization is the low temperature treatment given to water soaked seeds, slightly germinated seeds or seedlings to hasten the time of flowering of plants that will develop from them.

Chourad defined it as "acceleration of ability to produce flower by chilling treatment (1°-10°C) is called vernalisation".

Some important food plants, wheat, barley, rye have two kinds of varieties: winter and spring varieties. The ‘spring’ variety are planted in the spring and come to flower and produce grain before the end of the growing season. Winter varieties, however, if planted in spring would normally fail to flower or produce mature grains within the span of a flowering season. Hence, they are planted in autumn (September to October). They germinate, and over winter come out as small seedlings, resume growth in the spring and are harvested usually around mid-summer.

Vernalization is also seen in biennial plants. Biennials are monocarpic plants that normally flower and die in the second season. Sugarbeet, cabbages, carrots are some of the common biennials. Subjecting the growing of biennial plants to a cold treatment stimulates a subsequent photoperiodic flowering response.

REQUIREMENT OF VERNALIZATION

Low temperature : Low temperature required for vernalization is usually 0-4°C in most of the cases. The chilling treatment should not be immediately followed by high temperature (i.e., about 40°C), otherwise the effect of vernalization is lost. This phenomenon is called de-vernalization.

Duration of low temperature treatment : It varies from species to species from a few hours to a few days.

Actively dividing cells : Vernalization stimulus is perceived only by actively dividing cells, e.g., embryo tip, shoot apex and leaves. Therefore, vernalization treatment can be given to the germinating seeds or whole plant with meristematic tissues and other conditions.

Water : Proper hydration is a must for perceiving the stimulus of vernalization.

Oxygen : Aerobic respiration is also a requirement for vernalization. The stimulus has been named as vernalin (reported by Melchess, 1936-37).

SIGNIFICANCE OF VERNALISATION

Vernalisation shortens the vegetative period of the plant.

It increases the cold resistance of the plants.

SEED GERMINATION

When a seed of plant provides suitable conditions, various changes take place by which a seedling comes from seed and germinates, this is called germination of seeds. There is a requirement of water, oxygen and heat for the germination of seeds. There are various conditional factors inside the seed like food, hormones etc. for their germination.

The germination of seed takes place by following processes :

Hypogeal germination : At the time of seed germination cotyledons remain below the soil and micropyle absorbs the water by which integument breaks and radicle comes outside through micropyle by breaking the coleorhiza, root forms and shoot forms through plumule. This process of seed germination is known as hypogeal germination. E.g. pea, gram, maize, Cycas etc.

Epigeal germination : At the time of seed germination colytedons come outside from soil and seeds are fixed at the soil through secondary roots. This type of germination is called epigeal germination. E.g. pumpkin (Cucurbita maxima).

Vivipary : Viviparity is a unique adaptation in mangrove plants. Usually seeds require oxygen for germination. Oxygen is present in less amount in saline water. So, seeds germinate inside the fruit on the mother plant. This feature is called vivipary.

SEED DORMANCY

Dormancy may be defined as the inactive state of the seed in which the growth of the embryo is temporarily suspended for a specific length of time.

Many viable seeds germinate immediately after harvest if provided with suitable conditions of germination, i.e., water, oxygen and a suitable temperature (some seeds, e.g., lettuce need light also). However, perfectly viable seeds of many plants do not germinate immediately after harvest even when provided with suitable conditions of germination, i.e., their germination is blocked. This is called dormancy.

Seed dormancy may be due to many causes, such as external environment, endogenous control or conditions within seed itself some of which are as follows :

METHODS OF BREAKING DORMANCY

Chemical scarification : Treating the seeds with dilute acids, fat solvents, etc. Dormancy can be broken by treating the seeds with strong acids (N2SO4)or dipping in boiling water or rubbing on a rough surface.

High temperature treatment : Permeability of seed coat in alfa-alfa seeds increases when they are kept in water at the temp. of 85-90° for some time.

To neutralize the effect of inhibitors : Effect of germination inhibitors on the seeds can be counteracted by giving low and high temperature treatments to seeds or by treating the seeds with KNO3, thiourea, gibberellin, ethylene, chlorohydrin, etc.

Request a Call Back

Testimonials

Nothing beats the satisfaction of a job well done! Here are some testimonials from Clear Exam.

Clear Exam is an amazing institution. The teachers are really good, who can make your understanding of concepts, clear. They are ready to help you out whenever you need any help. In my opinion, this is one of the best institutions for JEE/NEET coaching out there.

- Mukul Sharma, IIT JEE One-year Classroom Program 2019

Joining Clear Exam is the best decision I have made to date. At Clear Exam you are not taught by teachers but by mentors who are highly proficient and technically grounded. Clear Exam is unique in every way imaginable; the teaching style is versatile making excellent use of the most advanced technologies while at the same time makes use of the most rudimentary examples to further the understanding of a concept. With Clear Exam, you don’t learn to solve the given problem; rather you learn to solve any given problem elegantly.

- Arpit Jain, IIT JEE Two-year Classroom Program 2020

Clear Exam is definitely a class apart from other coaching classes. The teachers are amazing, dedicated, and work hard to help us understand our basics. It has improved my school grades tremendously and has given me a different and broader outlook on many of the topics covered. I have thoroughly enjoyed all the classes so far, and look forward to the coming year!

- Taniya, NEET One-year Classroom Program 2019

Clear Exam has redefined the way I look at Science and Math. Rajan Sir and Jagdamba Sir have explained concepts really well and kept on stressing the importance of thinking logically and hence enabling students to build a strong foundation in these subjects. Classes have been fun.